In a large saucepan, sauté onion in butter until soft. Add curry powder and cook for another minute to blend the flavors. Add tomatoes and chicken stock. Bring to a boil and then reduce heat and simmer for 15-20 minutes. Add sour cream and let it melt in the soup. Whisk soup until smooth or puree in a blender. Garnish with a little dollop of sour cream and fresh parsley or chopped chives. You can also tuck a sprig of fresh basil beside the bowl or float a fresh nasturtium blossom.